Robots are designed to manipulate objects and interact with the environment. Safely detecting and controlling the forces that robotic actuators apply to robotic elements and/or the environment prevents unnecessary hazards. Traditionally, stiff actuators have been utilized to generate large forces/torques that can be measured by force sensors. Forces exerted by the stiff actuators, however, are difficult to measure precisely: small errors in the detected sensor deflections may result in large force errors. Therefore, expensive high-precision force sensors may be required to cooperatively work with the stiff actuators. In addition, stiff actuators are generally incapable of absorbing shock loads, thereby limiting their applications in a robotic system.
SEAs provide an economically viable approach to more accurately detect and control the robotic forces. SEAs typically employ a compliant element between a robotic gearbox and a driven load, as shown in FIG. 1, to reduce the stiffness of the actuator. Because the SEA has a relatively high compliance, its deflection in response to a force/torque is large and thus easy to measure. Force calculations based on the position deflection using, for example, Hooke's law thus have high accuracy. A highly compliant SEA reduces the sensitivity of the actuator to small changes in the position deflection of SEA; a feedback loop can be implemented to precisely control the actuator to a desired output force/torque.
A number of configurations, including torsion springs, extension springs, planar flexural elements and other elastic elements, have been used as the compliant element in SEAs. Torsion springs in general do not provide sufficient stiffness; deflections of the torsion spring may be large and result in errors in the force measurements due to the non-linear relationship between the force and the deformation for large deflections. Additionally, large deformations may significantly increase Coulomb friction and/or other non-conservative forces, thereby consuming extra energy. Extension springs typically generate significant friction due to sliding motion at the end regions of the spring. Planar flexural elements can provide the desired stiffness and element deflections; however, fabrication methods for these elements are usually expensive and the applied force may be distributed inhomogeneously thereon, resulting in a permanent deformation or fatigue failure of parts of the element.
Consequently, it remains a challenge to design a compliant SEA that is manufactured inexpensively, has limited energy loss (due to, e.g., friction) and strikes an optimal balance between a desired stiffness and a detectable position deflection while not exceeding the fatigue limit (or linear force regime) of the material.